1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus, a coil system for a magnetic resonance imaging apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having a Larmor frequency and reconstruct an image based on a magnetic resonance signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus, a coil system for a magnetic resonance imaging apparatus and a magnetic resonance imaging method which transmit a reception signal received by a RF receiver coil to a signal processing system by radio.
2. Description of the Related Art
MRI (Magnetic Resonance Imaging) apparatuses are used as monitoring devices in medical field conventionally.
An MRI apparatus is an apparatus which generates gradient magnetic fields in X, Y and Z axis directions by gradient coils on an imaging area of an object set in a cylindrical static field magnet for producing a static magnetic field, resonates nuclear spins in the object magnetically by transmitting RF signals from an RF transmitter coil and reconstructs an image of the object by using NMR (Nuclear Magnetic Resonance) signals generated due to an excitation.
The MRI apparatus like this employs a reception coil composed of a plurality of element coils for receiving NMR signals. Nowadays, the number of element coils included in the reception coil increases. However, the increase in the number of element coils in the reception coil increases cable wiring for transmitting reception signals obtained in the element coils to a signal processing system, and wiring of cables connected to moving element coils are subjected to various restrictions.
To address the problem of various restrictions on the cable wiring, a wireless coil system is proposed in which reception signals received by reception element coils is wirelessly transmitted to a signal processing system (see, for example, Japanese Patent Application No. 2005-505361).
To reduce labor of not only cable wiring for a reception coil but also other various kinds of cable wiring work, a technique employing optical communications (see, for example, U.S. Pat. No. 6,925,322) or wireless communications (see, for example, Japanese Patent Application No. H03-085145) is also proposed.
In addition, U.S. Pat. No. 5,384,536 discloses a wireless technique in an MRI apparatus, U.S. Pat. No. 5,245,288 discloses a wireless technique including a synchronous circuit in an MRI apparatus, and U.S. Pat. No. 6,791,322 discloses a wireless technique including a compression/extension circuit for improving a dynamic range by distortion correction in an MRI apparatus.
However, in known wireless coil systems, reception element coils are movable together with a table, whereas a receiver for wireless communications in a signal processing system is fixed. Therefore, the distance between a transmitter for wireless communications connected to each of the element coils and the receiver in the signal processing system varies depending on the position of the element coil, so that it is difficult to uniformly optimize the distance between the transmitter and the receiver. If the distance between the transmitter and the receiver is nonuniform, an SNR (signal to noise ratio) is decreased by signal attenuation accompanied by space propagation of a reception signal.
One approach to address the problem is a combined use of techniques of compression/expansion of an AM (Amplitude Modulation) signal, ACSB (Amplitude Compressed Single side Band), linear modulation, and/or other techniques to keep a sufficient SNR. However, this approach cannot avoid deterioration in a reception signal caused by signal compression with respect to a dynamic range required for an MRI apparatus and a complicated circuit structure.
Moreover, known wireless coil systems have no means for keeping a uniform spatial distance between the transmitter connected to each of the reception element coils whose arrangement varies with each imaging section, as described above, and the receiver in the signal processing system. Therefore, it is necessary to limit the arrangement of the element coils to suppress variations in the SNR. In addition, the variations in the SNR lead to image quality degradation.